A gas turbine engine, has a compressor, a combustor, and a turbine. An annular flow path extends axially through these components. Air entering this flow path is pressurized in the compressor, then mixed with fuel and ignited in the combustor resulting in a hot, high pressure gas. The hot gas is expanded across the turbine to produce useful work. A portion of this work is used to drive the compressor and the remainder is used to propel an aircraft with thrust or to drive a free turbine.
A rotor assembly extends axially through the engine and transfers work from the turbine to the compressor. In the turbine, the rotor assembly includes rotor disks having an array of rotor blades radially extending from the disks into the flow path of hot gas. These blades are angled with respect to the approaching flow of hot gas to extract work from the gas and to drive the disks about their axis of rotation.
A stator assembly circumscribes the rotor assembly. The stator assembly has an outer case which contains the working gas and has arrays of stator vanes. Each array of stator vanes extends radially from an outer endwall to an inner endwall crossing the flow path of the working gas upstream of an associated array of rotor blades. The stator vanes direct the working gas into the arrays of rotor blades at angles which optimize the performance of the engine. In order to reduce hoop stresses in the arrays of stator vanes and to make their manufacture and repair easier, it is common practice in the aviation field, to divide each array into discrete, circumferential segments.
The stator vanes are continuously exposed to the hot gas and therefore require cooling. One technique well known in the art for cooling stator vanes consists of forming hollow vanes having openings in both the inner and outer endwall and then taking cooling air from the compressor, passing it around the combustor, and through the openings into the interior of the vanes. In order to prevent the ingestion of hot gas into the structure surrounding the stator, the static pressure of the cooling air is necessarily higher than the static pressure of the hot gas. As a result, the cooling flow is drawn through and out the trailing edges of the vanes where it mixes with the hot gas. However, when this cooling technique is used with stators divided into segments some of the cooling air leaks through the gap between adjacent segments into the hot gas flow. This leakage mixes with the hot gas reducing its temperature which results in a significant drop in the performance of the gas turbine engine. Therefore, there is a need for a sealing apparatus and method that reduces the amount of cooling flow that leaks through the gaps between adjacent stator segments.
A prevailing technique for reducing this leakage flow is to machine a narrow, axial groove into the sides of both the inner and outer endwalls that border a gap. The grooves of circumferentially adjacent endwalls form a slot extending almost the entire axial width of the endwalls. A single, piece seal made of high temperature material is axially inserted into each slot for the entire length of the slot. These seals, generally referred to as feather seals or discouragers, inhibit the flow of cooling air through the gap because the pressure drop across the seals tends to force each seal to seat against the bottom of the adjacent grooves, thereby sealing the gap. However, because of dimensional tolerances inherent in all machining processes, adjacent grooves are unavoidably misaligned or mismatched in the radial direction. If the seal is not flexible enough to conform to this mismatch it will not seat properly against the bottom of adjacent grooves. A mismatch of only a few thousandth of an inch can result in considerable leakage around the seal.
The ability of the seal to conform to the mismatch of the slot and thereby reduce this leakage is directly related to its thickness. A thinner seal is not only more flexible but also will more easily sit against the bottom of the grooves when exposed to the pressure drop across the seal. However, if the seal is too thin it will buckle or deform when inserted into the slot. Consequently, the thickness of these feather seals results in less than optimum sealing.
Accordingly, a need still exists for a seal and method that is flexible enough to minimize the leakage flow through the gap and stiff enough to permit its insertion into a narrow slot without buckling or deformation.